Pacemaker catheter utilizing bipolar electrodes spaced in accordance to the length of a heart depolarization signal

ABSTRACT

A catheter lead for a cardiac pacemaker in which stimulating pulses are generated for delivery to the heart according to physiological need of the patient determined by signals obtained solely from the detection of naturally occurring P-waves propagating through the atrial myocardial tissue.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of my application Ser. No.333,085, filed Apr. 4, 1989, now U.S. Pat. No. 4,962,767 which is acontinuation-in-part of application Ser. No. 215,258, filed Jul. 5,1988, (now abandoned), and related subject matter is claimed inco-pending application Ser. No. 557,792, filed Jul. 29, 1990.

BACKGROUND OF THE INVENTION

This invention relates to cardiac pacemakers and, more particularly, tothe orientation and shape of the electrodes of a pacemaker catheter.

Cardiac pacemakers have been extensively used in patients with poorlyfunctioning heart pacing mechanisms due to breakdown in the cardiacelectrophysiological system. The pacemakers rectify malfunctioningsystems by stimulating the heart with electrical impulses and thuscontrolling the heart beat rate.

In normal functioning cardiovascular systems, electrical signals areproduced by the sino-atrial (S-A) node. The S-A node controls the heartbeat by stimulating the heart muscles through electrical signals ofsufficient magnitude and accurate sequential timing. The electricalsignals are conducted from the S-A node to the right and left atria, andare also transmitted from the atria through the AV node to the right andleft ventricles, which respond to the depolarizing wave and producecontraction of the heart muscle. Malfunction in the AV node conductionsystem between atrium and ventricle sometimes results in failure orblock of the transmitted signal. Simple cardiac pacemakers supply astimulus signal to the ventricle and thus cause the heart to beat at afixed rate.

Various other cardiac pacemaker system types have been used in the past,and the most widely used of these are discussed in U.S. Pat. No.4,365,639 to Goldreyer. One such system uses two catheters, onepositioned in the ventricle and the other within the atrium,respectively. The stimulating electrodes disposed on each of thecatheters were operated at a set rate, and with a time lag between thestimulating pulses. Several drawbacks to such a sequential pacing systemwere discovered, among them that two catheters were necessary forimplantation in the heart and that cardiac output was only augmented byabout 5 to 15% with fixed rate pacing. Also, two catheters were found tobe overly bulky and, furthermore, because the system only operated at arate fixed by the pacemaker, the heart could not compensate byincreasing the heart beat rate if the patient experienced increasedactivity.

Other systems relied on a sensing mechanism where the cardiac pacemakerhad both stimulating and sensing electrodes, or alternatively, one ofthe electrodes could act both as a sensor and as a stimulator. For thesetypes of systems, the sensor would be set to sense a certain type ofdistinctive electrical wave pattern, known as a P-wave associated withatrial depolarization, which is the signal sent to the heart muscle inthe ventricle via the AV node which causes the ventricle to contract. Ifa distinctive P-wave is sensed after a predetermined time interval fromthe last P-wave, the stimulating electrodes would then supply a pulse ofenergy of sufficient magnitude to stimulate the heart muscle intocontraction at the proper time in the heart beat cycle.

With these types of systems, however, it becomes increasingly importantthat the electrical signal be detected accurately and consistently.Several of the aforementioned systems, as well as others, haveemphasized the basic subject of endocavitary detection of cardiacsignatures of the electrical signals associated with cardiacdepolarization. Optimization of detection of signals in the atrialchamber of the heart is of particular importance because of the weaksignals obtainable from the traveling wave front along the atrialmyocardial wall during atrial depolarization.

Aforementioned U.S. Pat. No. 4,365,639 discloses a method to detectsignals in the heart by the use of electrodes normal (orthogonal) to theplane of the depolarizing wave in the atrial cardiac tissue. However,the configuration of the electrodes on the catheter, and especiallytheir shape and orientation in relation to each other, failed to takeadvantage of characteristics peculiar to the electrical signatures ofcardiac waves in order to more accurately and consistently detect thesignals indicative of a traveling wave front, such as a P-wave.Accordingly, it is an object of the present invention to improve thedetection of physiologic electrical signals by optimizing the electrodeconfiguration over that known to the prior art.

It is another object of the present invention to increase thesensitivity of a P-wave detection mechanism for providing a signalindicative of the passing of a P-wave to a cardiac pacer.

It is yet another object of the present invention to provide a catheterfor a cardiac pacemaker system which takes into account various factorssuch as the extracellular potential field dimensions, the propagationdirection of the field, anomalies in the conducting muscle fiberassociated with age, and the practical impedance level of the electrodesas it relates to the electrode surface area, as well as to otherfactors.

It is still another object of the present invention to provide a new andimproved electrode system which will provide for varying the heart ratein response to the body demand while at the same time utilizing only asingle catheter inserted through a vein, without necessitating openheart surgery. A particular object of the invention is to provide animproved sensing electrode arrangement for detection of P-wave signalsfor controlling the timing of ventricular stimulation.

It is also an object of the present invention to provide a catheter forsensing and for stimulation in the form of a single non-divergingfilament having two sensing electrodes mounted on the catheter withoptimal placement, shape, size and orientation of the catheterelectrodes.

It is a further object of this invention to provide for electrode sizesthat are small as practical relative to the dimensions of the width ofthe traveling wave front and small relative to the dimensions of thefield gradient normal to the wall of depolarizing muscle.

It is yet another object and a significant advantage of the presentinvention to optimize the parameters of the catheter electrodes in apacemaker system so as to increase the effectiveness of the cardiacpacemaker system.

It is also an advantage of the present invention to adjust theplacement, shape and orientation of the electrodes on a catheter of acardiac pacemaker system so as to tune the electrode parameters to thepeak negative to peak positive distance in the sensed traveling wavefront of the depolarizing wave indicative of the physiologic signalscreating a heart beat.

It is a further advantage and a unique feature of the present inventionto allow the adjustment of the catheter electrode parameters so as to besuitable for the individuals having a variety of ages, heart sizes andheart conditions.

It is another feature of the present invention to provide optimal size,placement and orientation of the atrial electrode array so as to fullytake advantage of the characteristics of the P-wave to improve sensingeffectiveness.

In accordance with these and other objects, features and advantages ofthe present invention, there is provided a catheter for use in a cardiacpacemaker system with electrode pair arrangements limited to twoelectrodes operating together in a direct or differential bipolarconfiguration for sensing cardiac depolarization in the blood pool andnot requiring contact with cardiac tissue.

The electrodes are so configured on the catheter relative to the cardiacdepolarization wavefront direction to prevent signal cancellation orexcess signal mitigation when the signals sensed from each electrode ofthe pair are electronically subtracted.

The electrode pair is configured so that the electrodes are placed onopposite sides of the catheter body but displaced longitudinally equalto or greater than the length of the depolarization signal along theaxis of the catheter and more or less parallel to the axis ofdepolarization to allow the combinational features of controlling theelectrode dimensions in all three planes to minimize field averaging inall planes and to prevent signal cancellation as the depolarization wavepasses each electrode.

The invention will be better understood and additional objects, featuresand advantages will become apparent from the following description ofthe preferred embodiments with particular reference to the drawingfigures, wherein like referenced numerals indicate like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a graph of an idealized extracellular wave formgenerated by the action potential as it propagates in the heart.

FIG. 1B illustrates a simplified model of a propagating extracellularpotential along the atrial wall.

FIG. 2 illustrates an embodiment of the device according to the presentinvention inserted into a patient's heart which is shown in a partialcut-away view.

FIGS. 3A and 3B show detailed side and end views, respectively, of theembodiment shown in FIG. 2.

FIG. 4 illustrates another embodiment of the device according to thepresent invention inserted into a patient's heart.

FIGS. 5A and 5B illustrate detailed side and end views, respectively, ofanother embodiment of the catheter electrodes according to theinvention.

FIGS. 6A and 6B illustrate detailed side and end views, respectively, ofanother embodiment of the catheter having the electrodes disposed onopposite sides of the filament.

FIGS. 6C and 6D show the catheter oriented in a 90° rotation from FIGS.6A and 6B, respectively, in relation to the atrial wall.

FIGS. 7A and 7B illustrate detailed side and end views, respectively, ofan embodiment similar to that of FIGS. 6A through 6D and havingelectrodes disposed as hemicyclical extensions substantially around onehalf of the circumference of the catheter.

DETAILED DESCRIPTION OF THE INVENTION

Theoretical electrical field theory regarding the signatures of thebioelectric phenomena associated with muscle or nerve depolarization hasbeen studied to better understand the propagation of electrical signalsthrough tissue membranes. Studies have shown that the electrode-sensedsurface potential along the axis of a depolarizing wave can be diphasicor triphasic, depending on the rate of change of rise and decay of thedepolarizing field. The decay rate, or repolarization of cardiac muscletissue, has been found to be slow so that electrode-sensed extracellularintrinsic deflections are normally diphasic. This diphasic nature of thenormal intrinsic signature can be advantageously employed in bipolarsystems to detect atrial depolarization by spacing, or tuning, theelectrode separation equal to the peak-negative-to-peak-positivedimension of the extracellular wave form.

FIG. 1A illustrates an idealized extracellular waveform generatedadjacent to heart muscle tissue by the action potential, usingapproximate dimensions. FIG. 1A is a graph of the extracellularpotential, measured in millivolts along the Y-axis 12, as a function ofthe longitudinal distance in the medium in which the wave form istraveling, as measured in millimeters (mm) along the X-axis 14. Foroptimum P-wave sensing, the cardiac pacemaker system includes circuitryfor the differential processing of the sensed signal. As the diphasicwave front passes a catheter electrode pair according to the presentinvention, the opposite peak polarities are added when the signal isdifferentially processed, thus providing a more accurate and effectivesensor of the passing extracellular potential waveform.

The present invention includes characteristics and elements tending totake advantage of the summation effect between the peak-positive 16 andpeak-negative 18 potentials so as to more accurately sense thedepolarization wave even when it is weak in relation to a normaldepolarization wave.

As shown in FIG. 1A, one peak maximum positive 16 and one peak maximumnegative 18 field signals occur in time if an electrode is placed in thewave front of the moving depolarizing wave, shown generally by referencenumeral 10. For purposes of this invention, the distance betweenmaximums peaks 16, 18 is defined as the one-half wave length of thesensed total depolarization wave, or the 2 mm distance traversed by thewave form, as shown by line 17 in FIG. 1A. This definition is usefulsince the wave form is not sinusoidal and not continuous periodic, as isthe usual case for transmission of radio-frequency energy through space.For instance, the upward slope 15 or 19 of the measured potential acrossthe electrode is less steep than that of the intrinsic deflection line17.

Referring now to FIG. 1B, the traveling wave front is represented bycross-hatching 22 traveling in the atrial wall 24 in the direction ofthe arrow Z. The wave front 22 creates an electric field in the spaceadjoining the atrial wall. This electric field is strongest close to theatrial wall 24, as is represented by the plus and minus signs marked inbold 25 and decreases with the distance from the point in the atrialwall where the depolarization is instantaneously occurring. The electricfield is attenuated by the medium in which it is shown, in this case,the blood contained within the atrium, and is represented as completelyessentially dissipating around a loosely defined boundary represented bydashed line 26. Definition of the various spatial directions may beunderstood from the representation of the X-direction 27 and Z-direction23. The wave front 22 travels in the Z-direction 23 and the field ismanifested in the X-direction 27 and Y-direction 29 (out of the plane ofthe paper).

It is also necessary to take into account that the traveling electricwave in a biologic system is to be sensed with highly conductive metalelectrodes placed within a volume conductor medium (i.e., blood or bodytissue) in which the field potentials are developed. The interfacebetween electrode and medium interface is quite complex and principallycapacitive in impedance but it is hypothesized that electrodes ofdimension that exceed the wave front dimension in the X and Z planes ofthe wave front distort the wave front potentials and averaging of thegradient along the axis and perpendicular to the axis of the wave frontoccurs, creating signal attenuation compared with a result that would beachieved with point source electrodes.

FIG. 2 illustrates the heart of a patient, generally indicated at 30,having a number of chambers, but principally the right atrium 33 and theright ventricle 35, shown in partial cut-away views, and the superiorvena cava 32, a major vein, connected to right atrium 33. The heart 30has a catheter 40 implanted through the superior vena cava 32. Thesino-atrial (S-A) node 36, at the blood fluid entrance to the atrium 33,depolarizes and initiates a depolarization wave front along the atrialwall 24, substantially as shown in the idealized model of FIG. 1B.

The catheter 40 comprises an in-line tri-axial connector 42, a single,flexible, non-diverging, coaxial insulated conductor 44 having twosensing electrodes 46, 48, a coaxial insulated conductor portion 47between electrodes 46, 48, and a distal electrode 50 at the tip of asingle insulated conductor 44. Detachment member 48', made of a flexiblematerial having flexible projections 49 projecting normal to anddisposed around filament 44 adjacent distal electrode 50, is provided tomaintain a stable position in the ventricle.

The catheter 40 is surgically inserted into the patient's heart 30through the superior vena cava 32, past the S-A node 36, and into theright atrium 33. The distal end, with electrode 50, is further insertedinto the right ventricle 35 through the tricuspid valve annulus 34. Inthe embodiment illustrated in FIG. 2, both sensing electrodes 46, 48 aredisposed completely within the right atrium 33, and the axis of thecatheter electrode pair 46, 48 is parallel to the direction of thepropagating action potential (Z-direction) in the region of theelectrodes.

Even when the electrodes are spaced along the axis of the lead, i.e.,essentially parallel to the general axis of depolarization along theZ-direction, another concern is the uncertainty of the conductiondirection and of the sensed signal morphologies in the region of theelectrodes, as the heart tissue is a non-isotropic medium. Theseuncertainties become more prevalent in the aging heart. The conductionvelocity of cardiac depolarization is known to be much higher in thedirection of the axis of muscle fiber compared to across the musclefibers. When this property is coupled with increased discontinuities inthe muscle associated with aging, the uncertainty of the direction ofpropagation in the region of an arbitrary electrode pair site increases.The signal morphology (i.e., shape and polarity of each signal) assensed on each electrode can also become complex in an anisotropicmedium, and additive signatures cannot be guaranteed.

For optimum detection of signals from healthy atrial myocardium, it ispreferable to space bipolar electrodes, such as 46, 48 closely together,i.e., tuned to the peak-negative-to-peak-positive dimensions of theextracellular wave form shown in FIG. 1A. For the aging heart, theuncertainties of propagation direction and signal morphology pointtoward an electrode spacing that exceeds the normal wave frontdimensions. The probability of achieving instantaneous additive signalson an electrode pair would diminish as the dimensions between electrodesincrease, but the probability of encountering subtractive signals wouldalso diminish. Thus the increased spacing of the electrodes in an agingheart is a worthy trade-off in a system where it is desirable to detectevery atrial depolarization cycle to maintain one-to-one atrialsynchronized ventricular pacing.

The subject of electrode size and shape for optimizing detection ofextracellular potentials is complicated by the practical reality ofhaving to connect the electrodes 46, 48, 50 through the triaxialconnector 42, and triaxial catheter 44 to a system in which the systemimpedance level cannot be guaranteed to remain high. Chronicallyimplanted pacemaker systems are subject to fluid entry into thenon-hermetic elements of the system. The connector header assembly andthe lead system used to connect the electrodes to the processingelectronics are both subject to the body fluid environments. Thematerials comprising the leads and connector header assembly elementsare permeable to fluid entry over longer periods of time, leading topotential fluid bridging across the various electrical interconnectingpoints.

Ideally, electrodes should be very small relative to the dimensionsbetween the peaks of the extracellular action potential because largeelectrodes average unequal isopotential lines over a large area. It ishypothesized that if electrodes instantaneously project throughisopotential lines of different amplitudes, the maximum amplitudeavailable from the detected extracellular signal would be reduced due toaveraging of the peak isopotential line with lines of lower amplitude.Furthermore, the frequency content of the signal generated on anelectrode as an extracellular wave form passes is related to both thelength of the electrode in the direction of propagation and theconduction velocity of the wave form. Stated another way, the actionpotential signal duration is artificially extended by the transit timeof the propagating wave over the length of the electrode.

The theoretically perfect bipolar electrode pair for sensing atrialsignals would be infinitely small individual electrodes with an axisbetween the pair oriented parallel to the axis of depolarization, andthe distance between these electrodes should be set to equal thepeak-negative-to-peak-positive dimension of the extracellular wave form.Further, each sensing electrode should be set directly on the atrialwall.

Unfortunately, this perfect configuration is not practical for placementon a pacing catheter and for dealing with the necessity of providing areasonably low impedance level needed for implantable pacemakerapplications. For such applications, electrode surface areas smallerthan about 4 mm² are impractical. Differential processing isparticularly sensitive to phase imbalances if chronic fluid bridging ofthe implanted system were to result in unbalanced loading of theelectrode pair. Also, because the electrodes are to be placed on thecircumference of a pacing catheter in which the angular orientationcannot be assured, the configuration choices to approach the idealbecome practically limited to ring electrodes or electrodes opposed andlongitudinally spaced apart on the catheter.

The embodiment shown in FIGS. 3A and 3B are two circumferential ringelectrodes which are shown in greater detail than in FIG. 2, and moredistinctly point out inventive aspects of the invention. The uniquefeatures of this ring configuration of electrodes include the dimensionsof and the spacing between the ring electrodes 46, 48. To optimizesignal detection, the ring electrode widths (D₁) should be as small aspossible consistent with a desire to maintain an electrode surface areaof 4 to 6 mm². It would, if possible, also be desirable to limit thering electrode diameter (D₂) to prevent averaging of the extracellularfield potential normal to the atrial wall 24 (X-direction). However,triaxial pacing catheters have practical limits of about 2 mm diameter.With a ring electrode diameter of 2 mm, the ring width has to be on theorder of 1 mm to provide an overall surface area in the 6 mm² range, thedesired surface area to maintain an adequately low source impedancelevel for use in the chronic implant environment.

The insulated filament portion 47 and spacing between the ringelectrodes (D₃) provide the benefit of tuning to thepeak-negative-to-peak-positive dimensions of the extracellular wave formwhich is optimally on the order of 2 to 3 mm. If intramyocardialconduction disturbances are suspected related to those encountered inaging cardiac muscle, the spacing between rings should be increased to 4to 5 mm to mitigate the potential of signal subtraction caused by acircuitous conduction path between electrodes and/or by abnormal signalmorphologies detected on each electrode related to propagation in ananisotropic medium.

Another inventive feature of the device as herein disclosed is tuningthe electrode spacing in the Z-plane, i.e., in the direction parallel tothe wave front, in order to match the peak-negative to peak-positivefields of the traveling wave front. The method of orthogonal detectionin contrast can only be optimized by spacing the electrodes at thelargest practical dimension across the lead body of the catheter.Orthogonal detection with bipolar electrodes measures the differentialgradient potential of the traveling wave front in a plane normal to thewave front. An orthogonal electrode pair can also be signal subtractivewhen processed differentially depending on the angular orientation ofthe catheter.

Referring now to FIG. 4, the general direction of normal antegradeconduction in the atrium 33 is from the S-A node 36 toward the rightventricle 35. Thus, it is inherently better to space electrodes 56, 58along the axis of filament 44, that is, parallel to this normalpropagation direction, rather than circumferentially around thefilament. The longitudinal orientation also allows greater freedom inchoosing an optimum spacing between electrodes 56, 58 because the choiceis not restricted to the filament diameter. In the circumferential(i.e., orthogonal) configuration, it is likely that signal substractioncan occur at least intermittently, since nearly identical extracellularwaveforms can pass the electrodes at the same instant in time, if bothelectrodes are equidistant from the atrial wall.

One embodiment of the invention using non-circumferential electrodes isshown in FIGS. 5A and 5B. This embodiment would be preferable to ringelectrodes if the angular orientation of the catheter filament 44 inrelation to atrial wall 24 could be maintained so that electrodes 56, 58remain adjacent the atrial wall 24, as shown. The smaller electrodedimension in the X direction (DX) minimizes averaging of unequalisopotential lines of the extracellular wave form normal to the atrialwall 24. The dimensions D₁, or length of electrodes 56, 58, should beminimized to prevent averaging of unequal isopotential lines in the Zplane. To maintain equivalency in surface area with the embodiment shownin FIGS. 3A and 3B, the dimensions D₅ and D₁ have to multiply to 6 mm².

The absolute optimum shape factor relies on a greater understanding ofthe electric field equations that define both the X and Z variables ofthe extracellular field shape. Of perhaps greater technical concern isthe effect the electrodes 56, 58 and the insulated filament 57 have onthe field shape. Like so many measurement situations encountered inscience, the measurement apparatus becomes a part of the problem to besolved. However, since the angle between the axis of the electrodes andthe direction of propagation cannot be known for certain, equal lineardimensions for D₅ and D₁ to achieve 6 mm² are most probably appropriate.The design to achieve a 6 mm² surface area requires about a 2.5 mm by2.5 mm linear dimension for D₁ and D₅. The choice for the dimensionsbetween electrodes 56, 58, i.e., (D₃), depends on the same argumentsused for the ring electrodes 46, 48 of FIG. 3B. That is, D₃ should beeither tuned to the action potential dimensions of 2 to 3 mm, or setapart greater than 3 mm to prevent signal cancellation. The difficultyin the embodiment shown in FIGS. 5A and 5B arises in maintaining theelectrodes 56, 58 next to the atrial wall 24.

FIGS. 6A and 6B show another embodiment being an optimal compromise ofthe embodiments shown in FIGS. 3A, 3B and FIGS. 5A and 5B. Thisembodiment takes into account the possibility of rotation of filament 44with respect to the atrial wall 24, and disposes hemicyclical electrodes56', 58' on opposite sides of filament 44. Like that of the ringelectrodes 46, 48, shown in FIGS. 3A and 3B, this configuration isrelatively forgiving of angular rotation of the catheter filament 44,since either one or both electrodes can be apposed to the atrial wall24. For example, if only one electrode is apposed to the wall,significant signal addition is unlikely because the second electrode isseparated from the wall 24 by the additional distance of the catheter 44in the X plane. The second electrode is probably also partially shieldedfrom the pertinent near field source by the insulated catheter filament57', which rests between the electrode 56', 58' and the atrial wall 24.

Another significant difference from the ring electrode configuration ofFIGS. 3A and 3B is that the dimension D₄ can be minimized as in theembodiment of FIGS. 5A and 5B so as to limit the averaging of unequalisopotential lines in the plane normal to the atrial wall 24, at leastin the angular position shown in FIG. 6A.

When the filament 44 is rotated 90°, as in FIGS. 6C and 6D, theadvantage of this embodiment compared to the ring electrodes of FIGS. 3Aand 3B is not so apparent. Signal summation, as described previously,can occur in this orientation, since both electrodes are equidistantfrom atrial wall 24. Signal addition would require that the dimension D₃be tuned to the extracellular wavelength of 2 to 3 mm. The need foroptimization of the electrode surface area and shape factor isessentially the same as in the embodiment of FIGS. 5A and 5B as is theneed for spacing the electrodes 56', 58' apart by a distance greaterthan 3 mm to account for anomalous behavior of the conduction system.

An even more optimal design may be the embodiment of the catheterelectrodes 56", 58" shown in FIGS. 7A and 7B. The practical constraintsof a catheter design, coupled with the need to interconnect the catheterto an implanted pacemaker (not shown) in a wet environment dictatesdesign principles that require compromise from the theoretically optimumconfiguration of Z-axis bipolar differential detection. Because of leaddiameter constraints, of practical electrode source impedance limits,and of lead angular placement uncertainties, the optimal design may beof hemicyclical electrodes 56", 58" having dimensions whichcircumferentially substantially encompass half of the circumference ofthe filament 44. In other words, dimension D₅ of either electrode 56"and 58" is substantially 180°, thus minimizing gradient potentialaveraging, yet maintaining the surface areas electrodes 56", 58" in the4 to 6 mm² range. This surface area represents a present practicallimitation imposed by pacemaker lead interwire impedances and theimpedances of the pacemaker lead/pulse generator interface at theconnection site resulting from the wet conductive environment.

For uses with acute instrumentation systems, however, the electrodesizes could be in 1 mm² range, provided the acute interwire impedancesof the filament were high, relative to the surrounding conducting mediumin which a reference electrode is placed. In the embodiment of FIGS. 7Aand 7B, if the filament diameter D₂ is in the range of 2 mm, anelectrode width D₁ of about 1.27 and a dimension D₅ in the hemicyclicaldimension of about 3.14 mm linear length around the filament providesthe minimum acceptable electrode surface area of 4 mm² for pacemakerapplications.

Although the present invention has been described with primary emphasison the preferred embodiments, it should be understood that variousmodifications can be made in the design and operation of the embodimentsdescribed without departing from the spirit and scope of the invention.For example, the first electrode 46 should be placed from about 10 cm toabout 16 cm from the end electrode 50 and the spacing of electrodes 46and 48 should be about 1 to 10 mm. The present embodiments are thereforeto be considered in all respects as illustrative and not restrictive,the scope of the invention being indicated by the following claimsrather than by the foregoing description, and all changes which comewithin the meaning and range of equivalency of the claims are thereforeto be embraced therein.

What is claimed is:
 1. A method for enhancing the detection of atrialP-waves using a catheter lead and a cardiac pacemaker in whichstimulating pulses are generated for delivery to the heart according tophysiological need of the patient determined by signals obtained solelyfrom the detection of naturally occurring P-waves propagating throughthe artrial myocardial tissue, said method comprisingimplanting a singlecatheter lead having a stimulating electrode at the distal end thereofand a pair of sensing electrodes thereon longitudinally spaced apart onopposite sides of the catheter from each other and from said distal endof the lead, by introducing the lead into the right atrial andventricular chambers of the heart via the patient's vascular system suchthat both of said sensing electrodes reside unrestrained within theright atrium with the lead substantially parallel to the tissue wallwhen the distal end of the lead is positioned at the apex of the rightventricle, measuring the approximate separation between adjacentpositive and negative peaks of an average P-wave for the recipient andlongitudinally spacing the sensing electrodes to approximate theseparation between adjacent positive and negative peaks of the naturallyoccurring average atrial P-wave for the patient in which the lead isimplanted, connecting the lead at its proximal end to the pacemaker sothat conductors in the lead separately electrically connected to saidstimulating electrode and said pair of sensing electrodes are therebyelectrically connected to the pulse generator and sense circuitry,respectively, of the pacemaker, and additively combining in the sensecircuitry of the pacemaker the respective signals on the separateconductors of the lead connected to each of said sensing electrodesresulting from detection of a P-wave, to enhance said P-wave detection.2. A catheter for use in a cardiac pacemaker system, said catheterhaving only an electrode pair arrangement limited to two electrodesoperating together in a bipolar configuration for sensing cardiacdepolarization signals without requiring contact with cardiactissue,said electrode pair axially configured on the catheter relativeto the cardiac depolarization wavefront direction, to prevent signalcancellation and excess signal mitigation when the signals sensed fromeach electrode of the pair are electronically processed by the pacemakersystem, said electrode pair is further configured such that theelectrodes are placed on opposite sides relative to the catheter's,longitudinal axis and displaced longitudinal along the axis of thecatheter and substantially parallel to the axis of depolarization toprovide for control of the electrode dimensions in three planes tominimize field averaging in said planes and prevent signal cancellationas the depolarization wave passes each electrode.
 3. A method forsensing cardiac depolarization in a cardiac pacemaker system comprisingthe steps, arranging an electrode pair of only two electrodes axially ona catheter, operating said electrode pair in a bipolar configuration,determining the cardiac depolarization signal length while the electrodepair is not in contact with cardiac tissue, configuring the electrodepair on the catheter relative to the cardiac depolarization signalwavefront direction to prevent signal cancellation or signal mitigationwhen the signals sensed from each of the pair of electrodes areelectronically processed by the pacemaker system, configuring theelectrode pair on opposite sides of the catheter and displacedlongitudinally a distance equal to the length of the depolarizationsignal along the axis of the catheter substantially parallel to the axisof depolarization, such that the combinational features of controllingthe electrode dimensions in each of three planes to minimize fieldaveraging in all planes prevents signal cancellation as thedepolarization wave passes each electrode.
 4. The method, as defined inclaim 3, further including the step of placing the electrode pair in ablood pool.